LectureOn Quantum Physics

Physics claims to be the science that studies
physical matter, so if we want to understand the nature of physical
matter, physics would clearly be one place to look. A very quick and
dirty (and grossly oversimplified) historical survey of what scientific
physics has told us about physical matter during the past several centuries
could be divided into three main periods: classical physics, molecular/atomic
physics, and quantum physics. Let's look briefly at what each period
has told us about the nature of physical matter.

Classical physics is
probably best represented by the work of Isaac Newton -- especially his Mathematical
Principles of Natural Philosophy. Classical physics has, for our
purposes, two simple principles, viz., that physical matter exists in
three separate states, solid, liquid, and gas; and that physical matter
in these three forms operates according to certain immutable physical
laws. That will be our too-simple summary of the basic principles of
classical physics on the question of physical matter.

The question to ask yourself now is whether
classical physics' description of how physical matter exists accords
well with your common-sense perception of matter. I think it does. This
is how physical matter actually looks and feels to us in our ordinary,
un-critically-examined daily experience.

Molecular/atomic
physics, the
next major stage in the historical development of physics, tells
us something very odd, however. It tells us that matter is not actually
the way it appears to us when we're using just our ordinary, uncritical
senses. According to this physics, matter is actually made up of extremely
tiny particles called molecules, and those molecules are themselves
made up of even smaller bits called atoms. Furthermore, according to
this physics, even those tiny atoms are made up of yet smaller bits
of matter called sub-atomic particles -- protons, neutrons, electrons,
and so on.

What we ought to notice as most odd, though,
is that the spaces between these subatomic particles are said to be enormous.
An average atom, as you probably know from your physics or chemistry
courses, is said to look a little like a miniature solar system. A central
nucleus (itself made up of subatomic particles) is surrounded by negatively
charged electrons that whirl about it in orbitals at huge distances from
the nucleus. To give just a rough idea of how far away from the nucleus
these orbitals might be -- and thus of how much empty space there is
in an average atom -- if the nucleus were the size of a baseball, the
first orbital might be several yards away from it, the next orbital might
be a few hundred yards away from that, and the next orbital a few thousand
yards away from that. The spaces between these circulating electrons,
therefore, are very large, so it would be accurate to say that the average
atom, like the average solar system, is made up mostly of space. It has
very little actual matter in it.

In fact, the proportion of matter (or stuff)
to space in the average atom is very small. One physicist estimated that
in the average atom (an "average atom" is clearly only a hypothetical
construct) the proportion of stuff to space is approximately the same
as if one baseball were suspended in the air in the middle of a large
baseball stadium like the Seattle Kingdome. One baseball in the midst
of all that empty space would be approximately the same proportion of
stuff to space that we would find in an average atom. So this means that
an atom, like a solar system, is mostly made up of empty space with a
relatively few very tiny particles circulating at great distances from
each other.

That's what atoms are said to be, according
to this physics, and molecules are just made up of bunches of atoms bound
together by certain forces. So any given chunk of matter, like your table,
for example, or your computer, or your elbow is made up of nothing but
gazillions of atoms all hanging together with each other. So if each
atom is almost all space, so is each molecule, and then so is the table
and your elbow and your chair and the roof and floor of the room you're
in right now. All made up mostly of empty space. Made up, in fact, almost
entirely of space.

That's what molecular/atomic physics teaches
us about what matter is really like. It basically says that yes, matter
does exist, but there really isn't very much of it in any given physical
object, and what you think of as matter is really almost all empty space.
Matter is nothing like what your senses tell you it is. It's almost all
empty space.

So let's ask now whether this account of what
matter is truly like fits well with your normal sensory experience of
the world. I think we'd have to acknowledge that no, this account of
what physical matter is like does not fit well with my what my ordinary
senses tell me. Our ordinary senses, according to this physics, do not
actually give us an accurate picture of what matter is truly like. Our
senses actually deceive us a bit here, probably because they are simply
not fine enough.

According to this kind of physics, then, physical
matter does exist, but there is not nearly as much of it as you might
have thought there was. The chair you're sitting on, which you may have
thought was solid matter, is actually almost all empty space, and so
is your own physical body that's sitting there in the chair. And so is
your shirt and the coffee mug and the mountains and the moon. All mostly
empty space.

And then along comes quantum physics, a very
strange bird indeed.

Quantum physics tells
us that all those little things that we used to call subatomic particles
are not actually truly particles at all.

What we used to call subatomic particles are
not exactly particles, or "things," at all. They are not so
much like little things, or little bits of stuff, as they are
like little energies. We might perhaps refer to them as little packets of
energy, except that that term might still give the impression that what
we used to call subatomic particles were still particles, or at
least were packages. To call them packets still might make it sound like
they are little bundles or parcels or items of some sort, and that is
precisely what quantum physics wants to avoid saying. So what we used
to call subatomic particles are now instead to be called little amounts of
energy. Just amounts of energy. The Latin word for "amount" is quantum (plural: quanta),
so we will now refer to these... these... well, what we used to call
subatomic particles, as quanta of energy, or amounts of energy.

So if we are going to speak accurately here
we will not strictly refer to these subatomic "particles" as "particles" any
more, because that would imply that they are things, stuff, matter. We
will instead refer to them as quanta of energy. And quantum physics,
at least when it is speaking strictly and precisely, does not want to
imply that these quanta are "things," in the sense of material
particles. And that is why today's physics and chemistry textbooks seldom
draw atoms any more as little miniature solar systems. They now instead
more often represent atoms as clouds of energies, concentrated in a more
dense nuclear center and again concentrated in more and less dense orbital
regions at some distances from the center.

Another question that we then need to ask about
these little quanta of energy is whether they are even "existents" or
not, namely, whether they are even something that exists. Quantum physics
hesitates to refer to these little quanta as existents, and even refuses
to say that they actually exist at some place at some time. Quantum physics
prefers instead to say that these little quanta are more like probabilities than
they are like actualities. They are said instead to have a tendency
to exist at some place at some time. Current representations of atoms
as clouds that are more and less dense in different atomic regions is
an attempt to show that these little quanta -- what we used to call subatomic
particles -- have only a given probability of existing in certain
regions at certain times. In other words, they should not be conceived
of as existents at all, but should rather be thought of as tendencies to
exist. They should not be thought of as stuff at all, but only as probabilities,
or tendencies, to exist.

This kind of physics does not seem to fit well
with our normal perception of things at all. If this account of "physical" matter
is actually true (as the quantum physicists say it is), and if our ordinary
senses give us quite a different picture of how things are, then we are
left with the question of which account of the physical universe we are
going to believe, that of the physicists or that of our common sense
experience.

In any case, we have, with quantum physics,
left behind the over-simple concept of physical matter as just simply
existing, and have instead come to see physical matter as made up only
of quanta of energy with tendencies to exist. (And what kind of
a thing is a tendency, anyhow?) If this account is true, then
our images of physical matter may be little more than a kind of sensory
illusion.

As Sir James Jeans, a prominent physicist earlier
in the 20th century, expressed this idea:

The more we learn about the nature of the
physical universe, the less it looks like a great machine and the more
it looks like a great thought.

That is, the more that physics learns, the
more it sees that the cosmos is not made up of simple physical matter
like a great piece of ironworks all fit together with physical levers
and material gears and wheels. The more we learn about it, the more the
physical universe begins to look instead like a great cosmos made of
thought-stuff. Or as Shakespeare (not a physicist) has said so aptly: "We
are such stuff as dreams are made on; and our little life is rounded
with a sleep" (The Tempest , IV,i,148).

So does physical stuff actually exist or not?
Quantum physics suggests that physical matter, at least in the normal
sense that we usually think of it, does not exist at all. There may be
quanta and energies and tendencies and probabilities, but, odd as it
might sound to naïve common sense, physical matter as we normally
think of it does not seem to exist at all.